1. Field of the Invention
The present invention relates to a micromachine and a method of fabricating the same, and in more detail, a micromachine having a frequency selection function and can be integrated with a semiconductor device, and a method of fabricating the same.
2. Description of Related Art
A micro-resonator fabricated based on a semiconductor process is characterized by its small device occupational area, capability of realizing a high Quality-factor, and possibility of integration with other semiconductor devices, and its use as an IF filter and an RF filter, out of various wireless communication devices, has been proposed by several research institutes including Michigan University (see non-patent document 1, for example).
The micro-resonator ever proposed and examined, however, has a resonance frequency only as high as not exceeding 200 MHz at maximum, and has been unsuccessful in providing its characteristic Quality-factor in a frequency range in GHz band, unlikely to a conventional gigahertz (GHz) filter based on surface acoustic wave (SAW) or a film bulk acoustic resonator (FBAR). At present, there is a general tendency of lowering in a peak of resonance frequency as an output signal in higher frequencies, so that improvement in signal-to-noise (S/N) ratio of the peak of resonance frequency is essential for obtaining a desirable filter characteristic.
According to a disk type micro-resonator described in non-patent document 1, a noise component in an output signal is ascribable to a signal which directly transmits a parasitic capacitance formed between the input and output electrodes, and it is described that the noise component can be reduced by disposing an oscillator electrode applied with direct current (DC) between input and output electrodes. On the other hand, a DC voltage exceeding 30 V will be necessary for the disk type oscillator to obtain a sufficiently large output, and a preferable example of a practical configuration will be such as having a beam structure using a Clamp-Clamp beam. One typical beam structure applied with the above-described noise component reduction method will have an electrode arrangement as shown in FIG. 6. In FIG. 6, a stacked film 114 composed of a silicon oxide film 112 and a silicon nitride film 113 is formed on a silicon substrate 111, an input electrode 115 and an output electrode 116 are formed thereon in parallel as being spaced from each other, and further thereon a beam resonator 117 is disposed across the input electrode 115 and the output electrode 116, while being spaced by a micro air gap.
[Non-Patent Document 1]
Clark T. -C. Nguyen, Ark-Chew Wong, Hao Ding, “MP4.7 Tunable, Switchable, High-Q VHF Microelectromechanical Band Pass Filters”, 1999 IEEE International Solid-State Circuit Conference, P.78–79
Arrangement of the input electrode and the output electrode shown in FIG. 2, however, still suffers from parasitic capacitances C1, C2, as shown in FIG. 7, which reside between the input electrode 115 and the output electrode 116 in a space therebetween or through the underlying layer (stacked layer 114). In particular for an oscillator of a gigahertz design, S/N ratio will generally degrade due to shrinkage of the structure and narrowing of the distance between the input and output electrodes. This demands further reduction in the parasitic capacitance between the input and output electrodes. The present invention is therefore to provide a micromachine having a reduced parasitic capacitance between the input and output electrodes, and ensuring a large S/N ratio even under operation in higher frequencies.